/*
 * Copyright (C) 2016 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.google.android.exoplayer2.trackselection;

import com.google.android.exoplayer2.ExoPlaybackException;
import com.google.android.exoplayer2.ExoPlayer;
import com.google.android.exoplayer2.Renderer;
import com.google.android.exoplayer2.RendererCapabilities;
import com.google.android.exoplayer2.RendererConfiguration;
import com.google.android.exoplayer2.source.TrackGroupArray;

/**
 * The component of an {@link ExoPlayer} responsible for selecting tracks to be consumed by each of
 * the player's {@link Renderer}s. The {@link DefaultTrackSelector} implementation should be
 * suitable for most use cases.
 *
 * <h3>Interactions with the player</h3>
 * The following interactions occur between the player and its track selector during playback.
 * <p>
 * <ul>
 *     <li>When the player is created it will initialize the track selector by calling
 *     {@link #init(InvalidationListener)}.</li>
 *     <li>When the player needs to make a track selection it will call
 *     {@link #selectTracks(RendererCapabilities[], TrackGroupArray)}. This typically occurs at the
 *     start of playback, when the player starts to buffer a new period of the media being played,
 *     and when the track selector invalidates its previous selections.</li>
 *     <li>The player may perform a track selection well in advance of the selected tracks becoming
 *     active, where active is defined to mean that the renderers are actually consuming media
 *     corresponding to the selection that was made. For example when playing media containing
 *     multiple periods, the track selection for a period is made when the player starts to buffer
 *     that period. Hence if the player's buffering policy is to maintain a 30 second buffer, the
 *     selection will occur approximately 30 seconds in advance of it becoming active. In fact the
 *     selection may never become active, for example if the user seeks to some other period of the
 *     media during the 30 second gap. The player indicates to the track selector when a selection
 *     it has previously made becomes active by calling {@link #onSelectionActivated(Object)}.</li>
 *     <li>If the track selector wishes to indicate to the player that selections it has previously
 *     made are invalid, it can do so by calling
 *     {@link InvalidationListener#onTrackSelectionsInvalidated()} on the
 *     {@link InvalidationListener} that was passed to {@link #init(InvalidationListener)}. A
 *     track selector may wish to do this if its configuration has changed, for example if it now
 *     wishes to prefer audio tracks in a particular language. This will trigger the player to make
 *     new track selections. Note that the player will have to re-buffer in the case that the new
 *     track selection for the currently playing period differs from the one that was invalidated.
 *     </li>
 * </ul>
 *
 * <h3>Renderer configuration</h3>
 * The {@link TrackSelectorResult} returned by
 * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} contains not only
 * {@link TrackSelection}s for each renderer, but also {@link RendererConfiguration}s defining
 * configuration parameters that the renderers should apply when consuming the corresponding media.
 * Whilst it may seem counter-intuitive for a track selector to also specify renderer configuration
 * information, in practice the two are tightly bound together. It may only be possible to play a
 * certain combination tracks if the renderers are configured in a particular way. Equally, it may
 * only be possible to configure renderers in a particular way if certain tracks are selected. Hence
 * it makes sense to determined the track selection and corresponding renderer configurations in a
 * single step.
 *
 * <h3>Threading model</h3>
 * All calls made by the player into the track selector are on the player's internal playback
 * thread. The track selector may call {@link InvalidationListener#onTrackSelectionsInvalidated()}
 * from any thread.
 */
public abstract class TrackSelector {

  /**
   * Notified when selections previously made by a {@link TrackSelector} are no longer valid.
   */
  public interface InvalidationListener {

    /**
     * Called by a {@link TrackSelector} to indicate that selections it has previously made are no
     * longer valid. May be called from any thread.
     */
    void onTrackSelectionsInvalidated();

  }

  private InvalidationListener listener;

  /**
   * Called by the player to initialize the selector.
   *
   * @param listener An invalidation listener that the selector can call to indicate that selections
   *     it has previously made are no longer valid.
   */
  public final void init(InvalidationListener listener) {
    this.listener = listener;
  }

  /**
   * Called by the player to perform a track selection.
   *
   * @param rendererCapabilities The {@link RendererCapabilities} of the renderers for which tracks
   *     are to be selected.
   * @param trackGroups The available track groups.
   * @return A {@link TrackSelectorResult} describing the track selections.
   * @throws ExoPlaybackException If an error occurs selecting tracks.
   */
  public abstract TrackSelectorResult selectTracks(RendererCapabilities[] rendererCapabilities,
      TrackGroupArray trackGroups) throws ExoPlaybackException;

  /**
   * Called by the player when a {@link TrackSelectorResult} previously generated by
   * {@link #selectTracks(RendererCapabilities[], TrackGroupArray)} is activated.
   *
   * @param info The value of {@link TrackSelectorResult#info} in the activated selection.
   */
  public abstract void onSelectionActivated(Object info);

  /**
   * Calls {@link InvalidationListener#onTrackSelectionsInvalidated()} to invalidate all previously
   * generated track selections.
   */
  protected final void invalidate() {
    if (listener != null) {
      listener.onTrackSelectionsInvalidated();
    }
  }

}
